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Species classification

There are advantages of using a method of classification that illustrates evolutionary relationships in a clade.
There are difficulties in classifying organisms into the traditional hierarchy of taxa. </aside>

Traditional hierarchy of taxa

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Cladogram

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Cladogram illustrating the relationships of organisms within groups of taxa known as clades. The vertical line (stem) at the base (bottom) represents the last common ancestor. The blue and red subgroups are clades, each defined by a common ancestor stem at the base of its respective subgroup (branch). The green subgroup alone, however, is not a clade; it is a paraphyletic group relative to the blue subgroup because it excludes the blue branch, which shares the same common ancestor. Together, the green and blue subgroups form a clade.

https://www.youtube.com/watch?v=CPcNfQfjjiw

Transcript

https://www.youtube.com/watch?v=YM6Ekb5De2o

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Difficulties with Traditional Taxonomic Classification

The traditional Linnaean system (Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species) organises life into nested categories based on observable traits. While useful, this approach presents several challenges:

Arbitrary boundaries: Evolution is a continuous process, yet taxonomy requires discrete divisions. There is no objective criterion for deciding when differences justify separate genera or families. As noted in the Crash Course video, there is literally one family (Formicidae) for over 12,000 ant species – a subjective decision made historically.

Misleading physical similarities: Traditional classification often relies on observable traits, which can group unrelated organisms together due to convergent evolution. For example:

Flight evolved independently in birds and insects (butterflies), so this trait cannot be used to show relatedness
Endothermy (warm-bloodedness) evolved separately in mammals and birds, yet crocodiles are more closely related to birds than humans are

Analogous vs homologous traits: Structures may look similar but have different evolutionary origins. A panda's "thumb" appears similar to a human thumb but is actually a modified wrist bone – an analogous trait that evolved independently.

Gene flow complicates boundaries: Species are not always distinct. Polar bears and brown bears can interbreed to produce "pizzly" or "grolar" bears. Horizontal gene transfer in bacteria creates web-like rather than tree-like relationships.

Advantages of Cladistic Classification

Cladistics groups organisms into clades – groups containing a common ancestor and all its descendants. This approach reflects actual evolutionary history rather than superficial similarities.

Based on evolutionary relationships: Cladograms show how organisms are related through common ancestry. Each branch point (node) represents a shared ancestor. Sister taxa share the most recent common ancestor.

Uses shared derived characteristics (synapomorphies): Rather than any shared trait, cladistics focuses on derived characteristics that arose in a common ancestor and were inherited by descendants. For example, retractable claws unite leopards and domestic cats in a clade.

Reveals unexpected relationships: Cladistic analysis shows that crocodiles are more closely related to birds than to lizards or snakes. Both are archosaurs sharing traits like four-chambered hearts, nest-building behaviour, and vocalisations – traits inherited from their common ancestor (and shared with dinosaurs).

More objective and predictive: Because cladistics uses measurable genetic and morphological evidence, it provides a more stable basis for classification. If we place an organism within a clade, we can predict it will possess the synapomorphies of that group.

Avoids misleading groupings: Traditional "Reptilia" was shown to be paraphyletic because it excluded birds despite their close relationship with crocodilians. Cladistics corrects such historical errors.

Niche requirements

The niche of a species can be defined as fundamental or realized.

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https://www.youtube.com/watch?v=0M0-3_m_qTw

Transcript